Location from harry-potter-chamber-of-secrets by
The Slytherin common room beneath the Black Lake -- where Harry and Ron, disguised as Crabbe and Goyle via Polyjuice Potion, interrogate Draco about the Heir of Slytherin and learn he doesn't know either.
Harry and Ron infiltrate the Slytherin common room by drinking Polyjuice Potion and transforming into Crabbe and Goyle. They have one hour before the potion wears off. The room is cold, green-lit, and hostile -- everything the Gryffindor common room is not. Draco holds court, bragging about his father's influence and hoping the Heir of Slytherin kills more Muggle-borns. But when pressed, he admits he doesn't know who the Heir is -- he just wishes it were him. The mission is both a success and a failure: they confirm Draco is not the Heir, but they're no closer to the truth. Meanwhile, Hermione is in the hospital wing with a tail and fur, having accidentally used a cat hair instead of Millicent Bulstrode's.
A long, low underground room extending beneath the Black Lake. Greenish light filters through tall windows into murky water. Silver serpents twine around pillars. Dark leather sofas cluster around a green-flamed fireplace.
Also known as: Slytherin Dungeon, Slytherin common room, the dungeons